1. Field of the Invention
The present invention relates to a dielectric capacitor and its manufacturing method.
2. Related Art
Recently with the widespread use of a portable data terminal such as a cell phone and the like, there is a growing need of nonvolatile memory. FeRAM (Ferroelectric RAM), MRAM (Magnetic RAM), phase change memory, and the like as well as flash EEPROM already manufactured widely have attracted the attention as next-generation nonvolatile memory. FeRAM, especially, is leading other next-generation nonvolatile memory in manufacturing.
DRAM, typical volatile semiconductor memory, is provided with a paraelectric capacitor as a memory element while FeRAM with a ferroelectric capacitor as a memory element.
The ferroelectric capacitor comprises a lower electrode, an upper electrode and a ferroelectric film between the electrodes in FeRAM. When the ferroelectric film and the lower electrode are processed in manufacturing a semiconductor, there is a fear that the upper electrode is damaged under the influence on the upper electrode caused by the process. To prevent this, an upper electrode is generally formed with the area smaller than those of a ferroelectric film and a lower electrode.
In many cases, it is difficult to process the materials used to form a ferroelectric capacitor. For this reason, if a resist film is used as an etching mask, this resist film recedes during the etching and disappears in the worst case with an insufficient etching selective ratio. If the resist film disappears, the ferroelectric film is damaged by etching and as a result, the electric characteristic of the ferroelectric capacitor is to be deteriorated. To solve this problem, an oxide film with which an etching selective ratio higher than that of the resist film can be obtained is conventionally used as an etching mask.
Hereinafter, a conventional method for manufacturing a ferroelectric capacitor in which an oxide film is used as an etching mask. FIG. 20 is a sectional view of a conventional ferroelectric capacitor C1, and FIGS. 13-19 are sectional views showing each manufacturing step.
[Step 1] An adhesion layer (Ti, 100) 2 and a lower electrode (Pt, 2000) 3 are formed by spattering on an oxide film (SiO2) 1 formed by CVD method. Next, a ferroelectric film (SBT: SrBi2Ta2O9, 2000) 4 is formed on the lower electrode 3 with a spin coat method. Further, an upper electrode (Pt, 2000) 5 is formed on the ferroelectric film 4 by spattering (FIG. 13).
[Step 2] After a resist film 6 is formed, the resist film 6 is patterned by using a photolithography (FIG. 14).
[Step 3] The upper electrode 5 is etched by using the resist film 6 as a mask. A dry-etching method is used here (FIG. 15).
[Step 4] The resist film 6 is removed by ashing with O2 plasma (FIG. 16).
[Step 5] An oxide film (4000) 7 is formed by CVD method. After a resist film 8 is formed on the oxide film 7, the resist film 8 is patterned by using a photolithography (FIG. 17).
[Step 6] The oxide film 7 is etched by using the resist film 8 as a mask. A dry-etching method is used here. And the resist film 8 is removed by ashing with O2 plasma (FIG. 18).
[Step 7] The oxide film 7 is used as a mask, and the ferroelectric film 4, the lower electrode 3 and the adhesion layer 2 are processed with a dry-etching method (FIG. 19).
[Step 8] An oxide film (3000) 9 is formed by CVD method (FIG. 20). The oxide film 9 protects the ferroelectric capacitor.
The conventional ferroelectric capacitor C1 is manufactured through the above steps from 1 to 8.
Since SBT, constituent for the ferroelectric film 4, is oxide, there is a fear that the electric characteristic such as a dielectric constant is to be deteriorated when SBT is exposed to a reducing atmosphere such as hydrogen gas. However, since the oxide films 7 and 9 are formed by CVD method as described above, it is difficult to avoid the generation of hydrogen gas. When the oxide films 7 and 9 are silicon oxide film, the chemical reaction in forming a film is expressed as follows.
SiH4+O2xe2x86x92SiO2+2H2
According to the conventional ferroelectric capacitor C1 and its manufacturing method, there is a fear that hydrogen generated during the film formation process of the oxide films 7 and 9 diffuse into the ferroelectric film 4, and hereby the electric characteristic of the ferroelectric capacitor C1 is to be deteriorated.
The present invention has been achieved in views of aforementioned problem. The object of the present invention is to provide a dielectric capacitor and its manufacturing method capable of showing a good electric characteristic by preventing an external substance such as a reducing element from. entering and diffusing into a dielectric film.
In the first aspect of the present invention to achieve the above object, there is provided a method for manufacturing a dielectric capacitor including from the first to twelfth steps as follows.
First, a lower electrode film, a dielectric film and an upper electrode film are formed to be piled up from the first to the third steps. A first mask is formed on the upper electrode film in the fourth step. A patterned resist film can be used as the first mask.
In the fifth step, an upper electrode is formed by removing the upper electrode film with the first mask as a mask by, for example, etching. And in the sixth step, the first mask is removed.
In the seventh step, a first barrier film covering the surface of the dielectric film exposed on the surface of the upper electrode in the fifth step is formed. In the next eighth step, a function film is formed on the first barrier film. When the function film is formed, the surface of the dielectric film is covered with the first barrier film. Consequently, the dielectric film does not have chemical influences from the substances generated as the formation of the function film or from the function film itself.
In the ninth step a second mask is formed on the function film and, at the same time, on the region including the one corresponding to the upper electrode. A patterned resist film can be used as the second mask, similarly to the first mask. In the tenth step, the function film and the first barrier film are removed with the second mask as a mask by, for example, etching. Then in the eleventh step, the second mask is removed.
In the twelfth step, the dielectric film and the lower electrode film are removed with the function film as a mask, and a capacitor part comprising the lower electrode, the dielectric film, the upper electrode, the first barrier film and the function film is formed.
As described above, the first mask is used as a patterning mask on the upper electrode while the function film is used as a mask to pattern the dielectric film and the lower electrode. This manufacturing method facilitates to pattern the upper electrode in different shape from the dielectric film or the lower electrode. Further, this manufacturing method enables to prepare an appropriate mask in response to the characteristic of each material and to expect the improved patterning accuracy.
Further, the thirteenth step can be added. In the thirteenth step, a second barrier film covering at least the surface of the capacitor part is formed. Therefore, even when a part of the dielectric film is exposed as the side wall part of the capacitor part, the exposed part is to be covered with the second barrier film in the thirteenth step. Even if another film is formed near the capacitor part in the following step, the dielectric film does not have chemical influences from the substances generated as the formation of another film or from another film itself.
Further, the fourteenth step can be added. In the fourteenth step, the second barrier film is etched back until the upper surface of the function film located on the uppermost of the capacitor part is exposed. By etching back, since the second film is removed from the upper surface of the function film, the vertical size of the dielectric capacitor can be kept small. This is advantageous for a semiconductor device provided with the dielectric capacitor to be made flat or highly-integrated. Also, the manufacturing process can be simplified.
In the dielectric capacitor and its manufacturing method according to the present invention, an oxide film can be used as the function film. When the oxide film is formed by CVD method, hydrogen might be generated during the formation process. In this case preferably, the first and second barrier films prevent hydrogen from passing through and reaching the dielectric film. Further preferably, the first and second barrier films consist of oxide tantalum to obtain this function.
Also according to the present invention, the upper electrode, the dielectric film and the lower electrode are formed so that: the contact area of the upper electrode and the dielectric film may be smaller than the area of the upper surface of the dielectric film on which the upper electrode contacts; and/or the contact area of the lower electrode and the dielectric film may be smaller than the area of the bottom surface of the dielectric film on which the lower electrode contacts. In the capacitor part, the region functioning as a capacitor (capacitor region) is the area between the upper electrode and the lower electrode. By forming the upper electrode, the dielectric film and the lower electrode as described above, the region which is not located between the upper electrode and the lower electrode (non-capacitor region) can be saved near the side wall part of the dielectric film. With this structure, even when an impurity such as hydrogen and the like enters and diffuses from the side wall part of the dielectric film to the internal part thereof, the electric characteristic of the capacitor part is not to be deteriorated unless the impurity passes through the non-capacitor region and reaches the capacitor region. In addition, if a barrier film preventing the entry of an impurity is formed at the side wall part of the dielectric film, a synergistic effect on the entry of the impurity can be obtained and the electric characteristic of the dielectric capacitor can be kept in good state.
In the second aspect of the present invention to achieve the above object, there is provided a dielectric capacitor comprising: a lower electrode; a dielectric film formed on the lower electrode; an upper electrode formed on the dielectric film; a function film; a first barrier film located at least within the region between the dielectric film and the function film; and a second barrier film covering at least the side wall part of the dielectric film. And the second barrier film with which the dielectric capacitor is provided is a sidewall provided on a capacitor part comprising the lower electrode, the dielectric film, the upper electrode, the first barrier film and the function film.
With this structure, the first barrier film can prevent the dielectric film and the function film from contacting with each other. Consequently, the dielectric film, the side wall part of which is covered with the second barrier film, does not have chemical influences from the peripheral films including the function film. As a result, the electric characteristic of the dielectric film can be kept in good state.
Preferably, the following method will be adopted to form the second barrier film as the sidewall of the capacitor part. First, a specific material is grown on the capacitor part and the periphery thereof. After that, the grown film is etched back until the function film located on the uppermost of the capacitor part becomes exposed. By etching back, the grown film on the function film is removed and the remained grown film becomes the sidewall of the capacitor part, that is, the second barrier film. The vertical size of the dielectric capacitor can be kept small by forming the second barrier film as the sidewall of the capacitor part. This is advantageous for a semiconductor device provided with the dielectric capacitor to be made flat or highly-integrated.